Preparation of diisocyanates



y 1964 J. G. LOWENSTEIN PREPARATION OF DIISOCYANATES Filed Dec. 17, 1962 United States Patent M 3,140,305 PREPARATION OF DIISOCYANATES Jack G. Lowenstein, Baltimore, Md, assignor to FMC Corporation, New York, N.Y., a corporation of Delaware Filed Dec. 17, 1962, SenNo. 245,132 11 Claims. (Cl. 260453) This invention relates to a process for recovering a substantially pure aromatic diisocyanate product from a reaction mass, and more particularly it relates to the recovery of toluene diisocyanate from distillation residues.

The phosgenation of non-vicinal aromatic diamines, such as diaminotoluene, yields the corresponding diisocyanates, which are used commercially in making urethane resins.. The diisocyanates are normally separated from the phosgenation reaction mass by simple distillation procedures. A portion of the reaction mass composed of high-boiling side-reaction products, including complex ureas, polyureas, polybiurets and isocyanurates, however, doesnot distill out. This distillation residue represents a lowering in yield of desirable product since it contains a substantial amount of chemically bound diisocyanate in complex molecules. Moreover, some diisocyanate product has to be left behind with the residues solely as a solvent to keep the mass fluid enough to dispose of easily. Sometimes 50% or more of the residual mass may be the valuable product.

In the past, these residues were either discarded or else treated separately to recapture lost diisocyanate values by time-consuming, low-yield processes. One such process, for example, requires heating the residues at high temperatures for long periods of time. Not only is the diisocyanate yield very low, but the severe heating process creates hard crusts of polymerized residue which cannot be readily removed from the distillation unit.

One object of this invention therefore is to furnish a method of utilizing these residues to obtain bound aro-- matic diisocyanates, without resorting to long, high-termperature recovery methods. Another object is to furnish a method for recovering toluene diisocyanate from plantprocess residues by a simple on-stream continuous treatment of the residues that can be used'in a commercial" process.

I have now discovered a means for recovering an aromatic diisocyanate from distillation residues remaining in the reactor after normal distillation of the diisocyanate, as prepared by phosgenating the corresponding aromatic diamine, by feeding the residue as a thin film into a still and removing as overhead vapors, a. diisocyanate-rich product. Toluene diisocyanate, which is a preferred reactant for the purpose of this invention, will be used to illustrate my novel method.

I have theorized that these residues, which contain a plurality of isocyanate groups, undergo a form of molecular rearrangement when spread out as a thin liquid film and heated under vacuum. A large amount of combined toluene diisocyanate will be released and will distill oil in a relatively short time and under mild operating temperatures. This new source of toluene diisocyanate is completely surprising since most of these residues had, in the past, represented a serious processing problem. The major portion of the distillation residue is made up of isocyanate polyureas and biurets, and dimerized di- 3,140,305 Patented July 7, 1964.

isocyanate. The following reactions presumably occur within the still, at distillation conditions:

Reaction 1 CH3 0 CH 5 CH3 OH:

H a H N -N N=C=O NH:

| l N=C=O O=C=N N=C=O N=C=O Isocyanate Polyurea Toluene Mono Diisocyanate isocyanate Reaction II CH3 CH3 0 O H g H N- N N I 3 I N=C=O N=C=O O=C=N Biuret ([LH CH5 N=C=O N 2 l N=C=O N=C=O Reaction III N=C=O CH3 CH3 N=C=O N?=O I 2 l 1 on, N=o=o O=C=N Isocyanate Dimer InReaction I, an isocyanate polyurea rearranges to one mole of toluene diisocyanate and one mole of intermediate isocyanate compound. Two moles of the toluene diisocyanateare formed in Reaction II from biuret and in Reaction HI from an iso cyanate dimer.

In other words, when the residue, in a liquid state from the diisocyanate distillation step, is spread or wiped thinly overa large surface area maintained at conditions under which toluene diisocyanate distills, certain components of the residue will break down to the desired product. The product is then taken off as overhead vapor.

Reference to the accompanying drawing will illustrate how the wiped-film still is utilized in this invention.

The drawing is a cross-sectional elevation view of a typical wiped-film still. v

Residue feed from the distillation unit contains sufficient toluene diisocyanate to keep the residue sufiiciently fluid. The feed was prepared by stripping the solvent,

' 3 tion temperature. Wiper blades 4 are rotated along a horizontal axis by motor 5. The residue is moved rapidly as a thin film along the inner walls of the still. The vaporized materials pass into a separation chamber 6 (a censpread it rapidly over the inner surface in a thin film, causing thereby a certain amount of turbulence. This turbulent film how is advantageous because it promotes heat transfer through the film. As a result, distillation rates are trifugal separator may optionally be placed between this high and residence times are minimal. The blades may chamber and the still). The vapor passes out of the be adjusted to permit any desired film thickness, usually separation chamber overhead through duct 7, and into between /gz and of an inch. The feed is whirled across a cyclone separator 8 to remove entrained liquids. Liqthe heated inner surface and non-volatiles eventually are uids removed from the cyclone are recycled to the feed forced into the exit chamber, along with vaporized matethrough line 9, While a substantially pure toluene diiso- 1O rials. Owing to the small depth and large area of the mass cyanate Vapor is reiiibyed hr gh line 10. This overhead being exposed to heat, reaction and subsequent volatiliza- Product y be returned to the main y nate dis ation occurs rapidily enough to permit continuous operation tion P- The lion-Volatile material leaves t ti of the still. In fact, this still can be operated simultaneousthrbilgh duct 11 and y be discarded repi'ocessedly with the main distillation unit. The bottom residues Other types of wiped-film stills, besides the tapered model, 15 fro th t i can b Continuously f d i h i having parallel walls, and either vertically or horizontally fil ilt Positioned, y be j as Conveniently ut liz i the The temperature of the vapor leaving the still is prefer- Praetice of this iiiVentiOIlably in the range of about 100 to 140 C.; an average yp Phosgehetioh Procedures, 1 Prefer to use evaporation rate of about 1.5 to 2.0 pounds of toluene ditechniques substantially as described in US. Patent No. isocyanate per h per Square f t per degree Fahrenheit 2,903,703 to Lfltolllette and Johnston, October 1959, should be obtained. The rate could be changed, if dewherein the initial contact between the reactants is made i by varying the temperature, pressure, m clearat a temperature in the range of about to eithet ance, and blade speed. I find that under these preferred on a batch or Continuous basis However, the distiiie' conditions, the overhead vapors will contain from 98% to tion residues can be obtained by using any other method 100% tohleme diisocyanate of p g i this p of e inventioh is y no The bottoms liquid is a viscous, tarry material which means limited to specific Phosgbhetioti Conditions generally has just enough fluidity to drain from the still Since the phbsgenatioh reaction is ordiheiiiy Performed by gravity. The rate of material flow through the still in Organic solvent such as mohochiorohehzehe t is enough to prevent the diisocyanate-thin residues from Vent is pp off before distilling the tbiuehe diisocya being baked onto the still surface. The temperature of mate. There is not limit to the amount of diisocyanate this material is in the range f 140 to 1 5 C and removed in the main distillation Step long as the 9 tains only about 25% isocyanate (calculated as toluene didue is maintained in a fluid state. The residual mater1al isocyanate, Substantially according to ASTM D1638 61T) is Pumped out the Prodhct Shh hf into wiped'hlm As illustrated in the tabulated data below, a large Preferably in a preheated Condition The wlpedhhh amount of valuable diisocyanate product can be recovered still is maintained under conditions suitable for distilling from the diisocyanate distillation residue diisocyanate t a rate which will hot leave the After reaction between phosgene and diaminotoluene, malhder of the residue too vlscou? to It most h in the presence of monochlorobenzene, the solvent carrier vantageous to use Y short resldfinqe tunes 1n thg Sun and trace amounts of phosgene were stripped off from the to prevent.undue tlnckenmg of i resldues reaction mass. The toluene diisocyanate product was The residue h may cpmam as 90% 180- then distilled in a distillation column under 10 mm. mercyahate by ahalysls fed 5 er a concury (absolute) at a temperature in the range of 120 to vement condltlons'u or excamp 6 es may e pr-e- 137 C. About two-thirds of all diisocyanate was reheated to about 80 to 115 C. prior to entering the still d u d h a :d t in order to bring it to reaction conditions in a shorter moved an c0 f T e Rmammg swimming time. The rate of introducing the feed will depend largely 40 about 845% lsocyahate (reported as toluene fhlsocya' upon the desired rate of removing overhead vapors at the hate) was hejated f fed to a one square P' other end of the still. I prefer to admit the feed at a rate evaporator With a Wiper bifide rotational Veiocity of i of about 20 to 80 pounds per hour per square foot of efp- All average film thickness of about /16 Of an Inch fective surface area in the still. The effective heating area Was used- T r sid time f the mass in the t ll of the still may vary as desired. ranged from 15 t0 30 Seconds- Run Number 1 2 3 4 5 6 7 Feed rate, lbs/hr 33 33 32 61 69 23 Temperature, C

Feed- 107 94 90 as 109 Jacket 168 171 153 156 154 15s VapoL- 126 127 117 116 114 Bottoms 160 162 141 141 132 151 Pressure, mm. Hg, abs. (throughout system)- 4.8 4.8 4. 6 4. 6 4.6 0 9 Percent Feed Evaporated 83.0 81.8 77.8 77.3 75. 9 82 9 Overhead Analysis:

Percent TDT 99.1 100.6 100.7 100.3 98.9 98.7 Percent Hydrolyzable Chlorine Group 0.01 0.0 0. 01 0. 01 0. 01 0. 03 0.04 Bottoms Analysis:

Percent Isocyanate (as TDI) .8 28.8 25.6 24.7 24.4 28.0 29.7 Percent Volatile Material .0 22.0 24.0 28.0 30.0 35.0 0.0 Percent Hydrolyzable Chlorine Grou .7 0.8 0.9 0.7 0.9 0.7 0.9

(The term "TDP above denotes toluene diisocyanate.)

Jacket temperatures can be varied to suit processing conditions. I prefer to keep jacket temperatures at from about to 200 C. Extremely high temperatures within the still may cause decomposition of the reaction mass. The still is preferably operated under vacuum, at 0.75 to 5 mm. of mercury. Higher temperatures and corresponding pressures may be used with some sacrifice of yield.

When the liquid feed enters the still, the wiper blades 75 The toluene diisocyanate so produced was a mixture of 80% by weight of 2,4-toluene diisocyanate and 20% of 2,6-toluene diisocyanate.

From the above data it may be seen that the conversion of distillation residues to toluene diisocyanate in the wiped-film still is remarkably high. The results show that a large amount of valuable product can be recaptured from an otherwise waste by-product.

Another advantage in utilizing my method of distillation is the improvement in handling the residues. The bottoms product, although rather viscous, can be removed from the still at the same time in which the desirable overhead product is evaporated. In conventional equipment, the residue would have to be kept in the still until the product is distilled. The removal of the bottoms product is performed while it is still fluid enough to leave the still by gravity.

This novel process applies equally to phenylene and other aromatic diisocyanates, including the position isomers, a swell as to 2,4-, 2.6-, and mixtures of 2,4- and 2,6- toluene diisocyanates. The presence of other non-reactive substituents on the aromatic nucleus should not atfect the recovery of diisocyanate complexes, formed during the phosgenation of the aromatic diamines, from distillation residues.

Having described by invention, I claim:

1. A process for the recovery of an aromatic diisocyanate from an aromatic diisocyanate distillation residue comprising passing said residue in the form of a thin film over a heated surface, thereby converting isocyanate-containing aromatic compounds present in the residue to an aromatic diisocyanate and distilling said aromatic diisocyanate therefrom.

2. A process for the recovery of an aromatic diisocyanate from a reaction mass residue comprising the steps of (l) reacting an aromatic diamine with phosgene and distilling off a portion of the aromatic diisocyanate so produced,

(2) passing the remaining residue into a second distillation vessel in which (a) the residue is distributed upon the inner surface of said vessel as a thin film, and (b) the film is subjected to a temperature and vacuum suflicient to evaporate the aromatic diisocyanate, and

(3) removing as vapors from said vessel, a vapor substantially rich in aromatic diisocyanate.

3. The process of claim 2, wherein the aromatic diiso cyanate is toluene diisocyanate.

4. The process of claim 3, wherein the residue is passed into the distillation vessel at a temperature in the range of to 115 C.

5. The process of claim 3, wherein the toluene diisocyanate is vaporized at a temperature in the range of 100 to 135 C.

6. The process of claim 3, wherein the residue contains up to toluene diisocyanate by weight.

7. The process of claim 3, wherein the distillation vessel is maintained under an absolute vacuum of 0.75 to 5 mm. of mercury.

8. A process for the preparation of toluene diisocyanate from a distillation residue comprising the steps of (1) passing into a distillation vessel from 20 to 80 pounds per hour of the distillation residue, obtained by (a) reacting diaminotoluene with phosgene, and (b) distilling off a portion of the toluene diisocyanate so produced,

(2) spreading the residue as a thin film upon the inner surface of said vessel by means of rotating wiper blades at an elevated temperature, causing toluene diisocyanate to form,

(3) distilling off toluene diisocyanate at a temperature in the range of to C., and

(4) discarding as a bottoms product, the non-volatilized substances.

9. The process of claim 8, wherein the bottoms product contains less than 30% isocyanates.

10. The process of claim 8, wherein the thickness of the thin film is in the range of to A of an inch.

11. The process of claim 8, wherein the toluene diisocyanate distilling 01f is selected from the group consisting of 2,4-toluene diisocyanate, 2,6-toluene diisocyanate, and a mixture of 2,4- and 2,6-toluene diisocyanates.

No references cited. 

1. A PROCESS FOR THE RECOVERY OF AN AROMATIC DIISOCYANATE FROM AN AROMATIC DIISOCYANATE DISTILLATION RESIDUE COMPRISING PASSING SAID RESIDUE IN THE FORM OF A THIN FILM OVER A HEATED SURFACE, THEREBY CONVERTING ISOCYANATE-CONTAINING AROMATIC COMPOUNDS PRESENT IN THE RESIDUE TO AN AROMATIC DIISOCYANATE AND DISTILLING SAID AROMATIC DIISOCYANATE THEREFROM. 